Conventionally, a vehicle lighting device includes a semiconductor light emitting device such as an LED (Light Emitting Diode) used as a light source. In an ON operation of the LED, for example, a battery voltage is chopped and raised by using a DC/DC converter as a power supply. The raised voltage is rectified and smoothed, and the smoothed DC power is supplied to the LED. In this case, a shunt resistor is inserted together with the LED in an output loop of the DC/DC converter, a current of the LED is detected from a voltage on both ends of the shunt resistor, and feedback control is carried out so that the current flowing to the LED is constant in the DC/DC converter based on the detected current. In the case in which multiple LEDs are connected to each other in series, each of the LEDs can be turned ON to generate the same amount of light even if a forward voltage Vf of the LED varies when the feedback control is performed so that the current flowing to the LED is constant.
On the other hand, in some cases when the LED is used as a light source, the LED is such that the current is greatly changed even if the supply voltage is varied slightly and a ripple component is superposed on the current flowing to the LED in the same manner as in a rectifying diode. For this reason, a smoothing capacitor having a large capacitance is used in the DC/DC converter to suppress the ripple of the current.
Moreover, it is possible to control the ON operation of the LED using the DC/DC converter by reducing a current (“If”) to be fed back, corresponding to the amount of a so-called extinction, when decreasing the amount of light emitted from the LED to carry out the extinction, for example. When changing the current to be fed back, corresponding to the amount of extinction, however, there is sometimes a problem in that a color shift is generated when the extinction is carried out to have a small light quantity of 10% with respect to a light quantity of 100% (a light quantity in a full ON operation obtained when a rated current is caused to flow to the LED), for example. More specifically, in an LED which emits white light through the supply of the rated current in the full ON operation, in some cases, when the current to be supplied to the LED is reduced in the extinction ON operation, a blue component in a luminescent color of the LED gradually is reduced and the LED emits a greenish color light.
Therefore, a method has been proposed in which a power circuit including a switching regulator. The method includes repeating control for supplying a rated current from the power circuit to the LED when a PWM (Pulse Width Modulation) signal for giving a command of the extinction of the LED is OFF (LOW level), and stopping the supply of the current from the power circuit to the LED when the PWM signal is ON (HIGH level) in response to the PWM signal. See Japanese Patent Document JP-A-2006-86063, particularly pages 3 to 6 and FIG. 1.) According to this method, during the extinction ON operation, a mean current flowing to the LED is reduced and the light emission of the LED is less than that in the a full ON operation. When the current flows to the LED, however, the rated current flows so that a white chromaticity of the LED can be maintained.
To carry out the extinction by approximately 10% in a full ON state by using the DC/DC converter, the ON duty of a switching device provided in the DC/DC converter is not simply reduced, but the switching device is inserted in series with an LED in an output loop of the DC/DC converter, for example, and the ON duty of the switching signal for ON/OFF controlling the switching device is set to be 10%.
If the switching device is inserted in series with the LED in the output loop of the DC/DC converter and the ON-duty of the switching signal for ON/OFF controlling the switching device is set to be 10% so that the ON operation having 10% extinction is carried out, there is a concern that an overshoot might be generated on the current (If) flowing to the LED the moment the switching device transitions from OFF to ON. It also is possible that the LED may not function because a capacitor having a large capacitance is provided on the output side of the DC/DC converter.
Specifically, when the output of the DC/DC converter has no load, that is, the switching device connected in series to the LED transitions from ON to OFF, an output voltage of the DC/DC converter is raised rapidly. When the capacitor having a large capacitance is used on the output side of the DC/DC converter, a large amount of charge is applied to the LED. Thus, an overshoot current flowing to the LED is increased when the switching device connected in series to the LED transitions from ON to OFF and is then turned ON again.
Operation of the DC/DC converter can be stopped while the switching device is OFF to reduce the overshoot current flowing to the LED. However, there is a concern that the output voltage of the DC/DC converter might be reduced during the stop operation of the DC/DC converter and that the current might not flow to the LED with the reduction in the output voltage of the DC/DC converter when the switching device transitions from OFF to ON.
When switching devices are connected in parallel with some or all of the LEDs connected in series, any of the switching devices connected in parallel with any of the LEDs as a light out target is turned ON to bypass current flowing from the DC/DC converter through the switching device, thereby turning OFF the LED and turning OFF the switching device connected in parallel with the LED to serve as the light out target. The current thus flows from the DC/DC converter to the LED, thereby turning ON the LED. A load of the DC/DC converter fluctuates significantly every time the number of the LEDs to serves as the light out target is changed. Furthermore, operation of the DC/DC converter cannot be stopped as long as the LED to serve as the light out target is present. Moreover, the load fluctuates with respect to the DC/DC converter. As discussed above with respect to the former circuit structure, therefore, an overshoot current is generated when the switching device transitions from ON to OFF and is then turned ON again.
In view of the foregoing circuit structures, it is possible to propose reducing ON speeds of the switching device connected in series with the LED and the switching device connected in parallel with the LED. That is, a transient ON state gradually increases the current flowing to the LED when the overshoot current is to be suppressed. However, a period for which a sufficient current does not flow to the LED is generated when the switching device is turned ON/OFF in an ON duty of 10% in order to reduce the ON speed of the switching device, for example, and the actual amount of light is reduced to 5%, for example, even if the ON operation with extinction of 10% is to be carried out. As a result, there is a concern that a linear relationship between the ON duty and the amount of light of the extinction might not be maintained.